Dual seismic surveying system



May 12, 1964 B. B. STRANGE 3,133,262

DUAL SEISMIC SURVEYING SYSTEM Filed March 2l, 1962 3 Sheets-Sheet 1 20 a; l l

deel/f f /A/VfA/ral' 500% ikv/7@ WM @P032 May 12, 1964 B. B. STRANGE 3,133,262

DUAL SEISMIC SURVEYING SYSTEM Filed March 21, 1962 5 Sheets-Sheet 2 www May l2, 1964 B. B. STRANGE DUAL sEsMIc sURvEYING SYSTEM 3 Sheets-Sheet 3 Filed March 2l, 1962 [MALL Y seismorneter-s.

3,133,262 t Palented May 12, 1964 ICC Y 3,133,262 DUAL SEISMIC SURVEYING SYS'IEM Booth" B. Strange, Shreveport, La., assignor to Western Geophysical Company of America, Los Angeles, Calif.

v l Filed Mar. 21, 1962, Ser. No. 181,270

12 Claims. v(Cl. 340-7) This invention relates to seismic survey equipment. and

,n more particularly to marine seismicl survey equipment, in-

cluding hydrophone cable and associated apparatus for determining the form of the ocean bottom and geophysical formations below the bottom of the ocean.

lnseismicsurvey work, it is customary to employe cable carrying a number of detectors which are sensitive toseismic signals. Those pick-up devices which are employed in marine work are known as hydrophones. ln land work, the detectors are known as geophones or In the usual apparatus, a set oct hydrophones is evenly'spaced'along the length of a iloating cable which is pulled along behind a ships A linear arrangement of detectors ofrthris type is known in the art as a spread A group of `detectors is sometimes employed instead of a single detector, and the phase hydrophone group or geophone groups or the simple term phone may be employed to designate either a single detector or Ia vgroup of closely spaced detectors. In operation, using the reection method, ian explosive charge is detonated in a hole in the earth, or in the water near the surface of the ocean at a point offset from the mid dle of the spread of detectors. i

The ship, in marine work normally stops brieflyrwhile the. detonation and recording `is accomplished. Shock Waves from the explosion travel through the Water to the bottom of the body of water and are reflected back to the hydrophones, and the resultant signals are recorded.

.When this conventional technique is employed, a profile, including a number of points corresponding to the number ofihydrophone groups, is obtained. The lengthof the profile Vis, normally equal to one-half of the overall spacing of the hydrophone spread, in View of the geometric pattern of the reflected waveswhich are picked up by the hydrophones. y v

Geological formations below the bottom ot the ocean may also be investigated through reflections at surfaces where the density or :other properties of the strataunderi go abrupt changes. Thus, for example, a salt dome which y might otherwise ybe obscured by layers of silt forming a level ocean bottom, is readily revealed by conventional oceanographic survey techniques` `One problem which plagues oceanographic surveyors is the false indications occasionally produced by multiple reflections. Such false reflections typically occur when Vthere is' ai sharp discontinuity both at fthe bottom of the ocean and at la dominant stratum below the bottom4 0f the ocean. Under these adverse conditions, seismic waves pass through the bottomy of the ocean, are reflected vfrom the dominant stratumV back to the bottom or snrace of the ocean; thengthe seismic waves are reileced downward again to the dominant stratum, and they are lreflected upward through the ocean :door to the hydro- Vphone array. Donble reflections of this type give a false indication of an additional discontinuity located below the dominant stratum by adistance equal -tothe spacin g between the ocean floor and the dominant stratum.

. Double or higher-order multiple reflections also frequentfor removing the effects of normal moveout from the signals received at the various hydrophones. These techniques involve the compression or displacement of recorded infomation Vlirom remote hydrophones so that `it is consistent with recordedV information from .the centrallyrlocated hydrophones ofthe spread. Following pmocessing to remoyegthe effects of normal moveout, parallelffvisible reproductionsof the recorded signals provide Ian accurate indica-tion of subsurface formations,

In the absence of njormal moveout correction, the data from each set of hydrophones form curved patterns corresponding to the diiferencein the lengths of the paths which the seismic waves follow;

Returningto the problem'of multiple waves, norm moveout correction teehniquesmay b e employed to reveal multiple reflections if the spread yof the hydrophones is suiciently large'with Irespect to the depth of the subsurface formations which 'are being studied. Thisis possible becanseof the difference in the velocity -of seismic waves at different Ydepths. Specifically, the velocity of seismic waves increases significantly with increasing depth. A multiple reflectionl echo from a discontinuityfnear the surface-thereilore has travelled (at a lower velocitythan a normal seismic wave which is reflected only once-from a deeper stratum.- The moveout is inversely related to the velocity; accordingly, the moveout of a multiple reflection signal signfiiicantlygreater than the moveout-tor a reilected signal from a corresponding true deeper substratum. Standard `arrangements designed to correct moveout at the apparent location of a deeper substratum would not fullyA correct moveout for multiple reflections, and would therefore provide curved. pattern for the talse echos, ifrrthe spacing of the hydrophones is suiciently great, When" the spacing of the hydrophones is fairly small with respect to the depth of Vthe, stratum underv investigation, however, there is very little moveout and even less difference in `moveout between true single reilections and false multiple reilections. The difference in the latter case becomes less than the ordinary random errors of measurement. The erroneous nature of the multiple reflection pattern, therefore, cannot easily be detected.A

Accordingly, for .the purpose orf .distinguishing andrejecting false patterns 'caused'by multiplel reflections, it is Vdesirablethat theV hydrophones be spaced over a long spread and hence, since the number ci groups is iixed, far apart. In order toprovide a moderately high density offpoints along the protile, however, it is desirable that the hydrophones be spaced fairly close together. I:Upto the present time,v these` conliicting requirements have usually been resolved kby using an intermediate spacing in which the hydrophones are neither as close together 4as would be desired fior detailed examination or :the conguration of the profile, nor as broadly spaced as would becdesinable froina standpoint of detecting and'rejecting false signals. In some cases separate successive studies from some'hydrophones in common with the other recorder and to receive other signals from hydrophone sta- Another more specific object of the inventionis the combination of closely-spaced and widely-spaced spreads of detectors in la practical and economical manner.

In accordance with the present invention, the foregoing objects may be realized by the use yof a compound hydrophonespread in which both a closely-spaced spread of detectors and a widely-spaced spread of detector are included. By choosing the spacings of fthe longer spread to be a multiple of that of the shorter spread, many of the detectors in the `complete linear system may be used in common On board the ship towing a cable carrying the compound spread of hydrophones, two recorders may be provided to record signals from the long hydrophone spread and from the short hydrophone spread, respectively. When this arrangement is employed, the output signals for some of the hydrophones are connected to both recorders.

In one specic implementation of the foregoing arrangement, a central section of 24 closely-spaced hydrophones or hydrophone groups were provided for the closelyspaced spread. In addition, twoend sections of cable each including six widely-spaced hydrophone pick-up stations are'connected to either side of the central cable section. These 12V, additional hydrophones, together with every second one of the hydrophones inthe central section of the cable provided the broad-spaced spreadof 24 hydrophones. With .this arrangement the central section of cable may be employed alone where physical conditions do not permit the use of the long cable or where, for other oceanographic mapping reasons, a long array is not needed. However, for general surveyV purposes where it is highly advantageous to have data from both the long and the short spreads, the full cable is employed, with l2 of the hydrophone stations being used in common between the long and the short spaced arrays. Under these conditions, two 24 track recorders "are employed, one for recording information from the closely-spaced spread, and one for recordinginformationfrom the other spread in which the hydrophone pick-up stations are spaced relatively far apart. f f

As noted above, the distance along the reflecting surfaces covered by each shot is approximatelyone-half the length of the spread of hydrophones. In the present case where the short spread is one-half the length of the active portion of the complete cable, shots are made with an explosion near the center of the cable whenever the ship moves a distance equal to one-quarter the total length of the cable. While both recorders may be activated during each shot, it is only necessary to energize the recorder associated with the longer spread during every other shot. This follows, of course, from the double length-of the longer' spread in the specific illustrative example set forth above. v

In accordance with a feature of the invention, therefore, an oceanographic seismic survey cable may be provided with a first spread of hydrophone pick-up stations spaced apart by relatively short distances along the length of the cable, and a second spread'of hydrophone pick-up 'stations or a multiple of the short spread, so that some of the hydrophone stations may be used in common.

V spaced apart by distances which are commensurate withV Separate recorders may be provided for the two spreads;

with each recorder being connected to receive signals tions to the exclusion of the other recorder.

In accordance with a featured method of the present invention, oceanographic surveying may be accomplished by simultaneously recording from a closely-spaced spread and a widely-spaced spread of hydrophones.

In accordance with another feature of the invention, the recording as set forth in the preceding paragraph may be accomplished by including at least some hydrophone stations in both the closely-spaced spread and the widelyspaced spread, and recording'information from these hy- Vdrophone stations in combination both with other hydroinclude reduction in equipment, in that some of the hydrophones required for two complete spreads may be eliminated, and also include obvious savings in explosives and time. and with a short spread simultaneously, the two surveys will assuredly cover precisely the same geographical area; evaluation of the resultant data is therefore greatly simplified.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of construction and operation, together with further objects, features and advantages thereof, will be better understood from the following description considered in conjunction with the accompanying drawing in which illustrative embodiments of the invention are disclosed, by Way of example.V It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only and does not constitute a limitation of the invention.

In the drawing:

FIG. l is a diagrammatic showing of the operation of 'oceanographic surveying equipment;

FIG. 2 is a typical showing of traces recorded from oceanographic survey Work;

FIG. 3 shows a compound hydrophone spread in accordance with the invention, and certain multiple reflections which may cause false indications-in oceanographicV it. The large reel 16 is provided on the rear deck of the vessel 12 to hold the cable 14 Vwhen surveying work is not in progress. the body of water which is being surveyed and the line 2i) represents the boundary between strata of material below the bottom Vof the ocean. The cable 14 extends to point 22. Twenty-fourv hydrophones are envenly-spaced along the-cable 14 at the points marked by xs between the points 22 and 24. The hydrophones are connected by individual cable pairs to recorders on the survey vessel 12.

In operation, a charge of explosives or a shot is detonated just below the surface of the water at a point a few hundred feet oset from the center ofthe spread of hydrophones on a perpendicular bisector of the hydrophone spread. The shot point is indicated at 26 in FIG. l.

The seismic or shock wave from the detonation-is shown in FIG. l by aV series of rays 28, 30, 32, 34 and 36. The rays 2S and 3i) are indicated as being picked up by the hydrophone located at point 22. It may be noted that In addition, by making surveys With a long spread l The line 1S represents theV bottom of `.the.ray v28`i-is reected fromlthebottom 18 of theocean whi1e the ray 30 is reflected'from the discontinuity 20. .Thepath of the shock wavefalong ray 28 is much shorter than that of the ray 30 which is reflected from the substratum. The hydrophone 'at point 22 will therefore pick rup an initial shock wave when ray 28 arrives', and will pick Uup a subsequent shock wave-when the signal from the substratuml arrives along path 30. ".These successive shock Waves produce electrical signals which are transmitted from thehydrophone at point 22 through cable 14 to a frecorderin Vessel 12. Y f VSimilar signals are picked up at each of the 24 hydro- "phonesv and the signals are recorded concurrently, usually "verted to optical form to give 'avisual yrepresentation of the subsurface geophysical '.Structures, as shown in ;FIG. 2.

Y i' Rays 32, y34, 36 are -all reectedfrom the discontinuity .lf-'20.Ray F32 follows arelatively short path fromA shot jpointi26 tofaV nearby hydrophone 38; ray 34V follows a located at point .24. The difference in timefor the shock waves to'reach hydrophones located'away from the middle I"ofthespread is termed normal moveout. Compensation for normal moveout may be achieved by the^dis placement )in time of thesignal received at the-more 'Y 1f distanthydrophones. Corrections accomplished by rela- /tiverdisplacement of the signals recorded at the hydro- Aphones areonly elective for signals reflected from a prefd'etermnedIdepth, however.

.. Vith reference to FIG. l, 'itIniaybe seen that the'total l istance which is surveyediis equal Yto S/2,''or"one`half A of thelength` of the hydrophone spread. vThis may be `observed" from' thepoints where rays 30 and 36 are Yre- "fleictedfrom the stratum 2l). 'The distance between these two'points is'designated S/ 2. at the bottom of FIG. l.

` "Lf FIG; 2 is a representation-'of the subsurface'features asnproduced by aseries of shots. In FIG. Z'the surfaceof the ocean is indicated by thev lines 42, 42'; the

`jbottomof the ocean is Vindicated by lines 44, 44'; anda ',fdiscontinuity which actually turned out tobe the upper v fsurface of 'a salt dome, is indicated bythe lines 46,462 l 'The top'of thesalt. dome was, of course, off to the right KL'of the proiile 'shownin FIG.` 2.

,The representation of FIG. 2 is formed by a series of i 'f theefshots centered atpoints 48, Si) and 5l; The bracket V`adjacent point 48 indicates the twenty-four recording' traces provided by`the .twenty-four hydrophones ener- 'gized when the first shottook place. Successive additional groups 24 traces a're indicated by the additional brackets associated. with points 50, and 51. The direct i shock wave which' passes horizontally through the water .fis indicated immediately under p oints 43, 50j and 51 .-'near ..the. top of FIG. 2. In each case the traces are formed;photographica1ly by blackeningofthe areas encompassed by the displacement of a line of light from its :neutral position; accordingly, 'the darkened areas p representthe zones where strong reflections have been received at the hydrophones. Moveout correction has beenv accomplished only for depths substantially below bottom of the ocean; thus,'the curved pattern of normal moveout is clearly. visible along lines 44, 44

representing theV bottom of 'the' ocean.

: f f Now that conventional 'oceanographic survey techniquesfhave been reviewed, reference is made to FIG. 3 showing `a compound hydrophone array in accordance with the' present invention.f .In the array of FIG. 3, a long spread-o fghydrophones is indicated by the xs located between points 52 and 54,1`he long spread extends for 3a, distance designated S in FIG. 3. A shorfhydrophone array, extending over a distance designatedfS. in FIG. :1"3, .includes the hydrophones designated x between pointsv 56and 58.'. .It also includespthe hydrophone groups -o which appearbetween sone ofthe hydrophone stations designatedfx, which' also form part of the'longer hydrophone array. In summarizing, therefore, the hydrophone i on. parallel tracks. These electrical signals may be con- .tween thel two `arrays lare designated by an o Other features shown in the vdiagram of FIG. y3 111-.

clude the bottom of the ocean designated 60 Vand two Vinterfaces between geologic strata 62 and 64. When a shot is detonated at point 66 near'the common center of both spreads, reliections Vare received fromthe deeper interface 64 at the remote hydrophones '52. :and 54, as -indicated by lin'es 68 and 70, respectively.' ',-Reections are received also,v at near hydrophone' 53:,A from point 77 on interface 64.

Multiple` reflections along the paths 72 andv 74 could provide signals at they hydrophones giving a false-indi'- cation of the presence of a substratum'approximately at the depth indicated by line 64"'. However,` as discussed. in the introduction, the velocity of seismic wavesincreases vsignificantly with increasing depth. Accordingly,.

with referenceV to FIG. 3, the waves followin'gvthe path '72; from .shot point 66 to the 'hydrophone at point 52 would travelmore slowly than waves traversing the lpath shown by .lines l68, even though the waves traveling .exactly the same elapsed times. The moveout, representing' the dilferencein time for the travel ofwaves to re- .mote hydrophonesSZ, 54 as compared with the elapsed time for. waves to be reflected to hydrophone. nearest the shot point 53,:.will ytherefore be greaterfor the multiple reflectionpath'72 than for the direct reflection pathY 68.

When normal moveout correction is madepfor thefindicated depth of' stratum `64', therefore, false indications involving multiple reflections are revealed by "the abfore'appear as a series of arcsrof sence of vfull correction for moveout. They would therethe typeshown along lines 44, 44';in FIG. 2. y v

When theA depth of the formation being surveyed is v moveoutoccurs. Accordingly, with reference to FIG. 3,

the moveout obtained from the spread S would be much less; than that obtained through the use of they spread S.

i When there is very little moveout in the original received set Vof signals itis most difficult to distinguisherroneous signals caused, by multiple reflections from true reflections from subsurface strata. Accordingly, it is frequently desirable to use a long `spreadsuch as that indicated by the spread gticularity. Obviou sly with 'a longer spread, a much lesser number of points per unit distance may be obtained. The

present compound hydrophone spread, with the shorter spread S .in addition to the longer spread S meets both 1 of these two requirements. Thus, it has the long length required for detecting spurious` multiple'reliections, `and also has the closely-spaced spread of hydrophones Vre-y quired for detailed .under-water surveys.

`. The compound hydrophone arraylishown inFIGfS .1 is arranged so. that the longer set of hydrophones has `its` center at the same point as the shorter spread of hydrophones. A single shot near the center of thev two spreadsis therefore employed to energize both spreads.

It is to be understood that variations-.in the arrangement could, vof course,.be employed. Thus, every third or every fourth-,hydrophone inV the central array could abe used in combination with the longer array. In addition, the Vshort array S could be used in combination t Vwith a longer set4 of widely-spaced hydrophones all lat one end. Thislast proposed array is designed to be used with shots yat the end of the cable rather than atthe center. vSuch arrangement `wouldstillenjoy thev'advantages.

S extending between pointsSZ and 54 in vention. The outermost cable section 82 carries. six hy- 24 which are applied to leads `143. Similarly, leads 146 for recorder 96 areconnected to all of the cable pairs 7 v through 30. VThe successive leads 7 through 30 provide drophone groups, as indicated by the extension of the spread S beyond the spread Sin FIG. 3. Cable section 'S2 therefore includes six'pairs of wires to carry signals from the hydrophones to the other cable sections 84, 66, E26 and 96, and ultimately tothe connection network 9. The additional cable sections S4, 86 and 33 have 18, 30, and 36-cable pairs,'respectively, to accommodate the additional hydrophones, aswell as signals from hydrophone groups inV cable sectionsrremote from the connection network 92. The cable pairs.V are brought out to the connection network 92, and signals from the appropriate leads are'rconnected to recorders'94 and 96. Recorder 94 receives signals from the longer spread S, while recorder 96 receives signals from the shorter spread S. As discussed previously, some hydrophones located .at common group centers provide signalsforv both record'- ers 94 and 96, while other hydrophones provide signals only for the recorder 94 associated with the longer spread S or for recorder 96 associated with the shorter spreads. Y

The cable of F164 is intended for towing behind a ship and may have each hydrophone connected to it in Vaccordance with known practices. Two known prior art arrangements are disclosed, for example, in L. C. Paslay Patent No. 2,465,696, granted March 29,1959, and I. H.

' VWoodworth PatentNo. 2,923,916, grantedFebr'uary 2,

1960. The hydrophones are-preferably located -at a uniform depth below the surface of the water. The

hydrophones Vmay also be of a known design,Y and may `centerline 104. Three wires 126, 128 and 130 are rassoelated with each group center. The hydrophones 112 and 114 are Vconnected in parallel between leads 126 andlt.V Lead 126 is common land lead 128 is normally the active color-coded lead on which the seismic signals from phones 112 and' 114 are sent. Lead 130 is a spare lead. Access for testing and changing .to spare leadspis provided by connections 132 and 134. Addible.

the twenty-four traces 1 through 24 which appear upon i leads V146 to produce the short spread signalsf The significance of the twenty-four sets of leads desig` nated by the reference numeral 146 and the additional twenty-four sets of leads 148 will be more readily understood by reference to; FIG. 2. As mentioned above, each of the groups of traces 48, and 52 in FIG. 2 include twenty-fourY traces which were derived from'a single shot. These twenty-four traces were produced by signals picked up at twenty-four spaced groups of hydrophones in the manner-described above. It should be Vnoted in passing that PEG. 2 represents a display derived from a single one the spacingbetweenV hydrophone group centers in the longer spread is 100 meters, or about 328 feet.

It is to be understood that the above-described arrangements are illustrative or" the application of the principles of the invention. Numerous other arrangements may be derived by those skilled in the Vart without departingL from the spirit and scope of the invention. Thus, by way of example and not of limitation, the longer spread can be three or four times the length of the shorter spread instead of merely double/the length of the shorter spread; similarly, three or kmore spreads can be included in a single cable, instead of just two spreads.V In preferred embodiments of the invention, the different spreads usev somecommon hydrophones, and the spacing for the detectors in the different spreads are` therefore commensura- For purposesV "of ineline shooting, the Vcloselyspaced hydrophones are-located toward one end of the cable` andthe more widely-spaced hydrophones are lo-V cated towar'd the Vother end; and other minor modifications oi' the structural and electrical interconnections of the proposed system can also be clearly accomplished. Thus, in actual practice, the cable of FIG. 4 can be divided into shorter sections for ease in fabrication, handling andrepair; It is also to be noted that,while the present invention is principally applicableY to marine seismic work, Lthe l concepts are also applicable to multiple geophone spreads tional hydrophones may Valso be provided at these. points.

FIG. 6 shows the electrical connections which are included in the network 92 of FIG. 4. As noted above, the

cable 90 includes 36 cable pairs corresponding to the number of hydrophones in the two spreads S and S. Wires 142 at the right-hand side of FIG. 6 are connected to cable 90; Each of the wires 142 actually represents'a Vcable pair rather .than a single line. The leads 142 are numbered from 1 to 36 to indicate ythe pair number and the Vhydrophone group to which the pair is connected.

Pair No. l is connected to the hydrophone group along the cable 88 which is closest to the ship, and pair 36 is connected to the farthestV hydrophone. The leads 142 are S' as shown in the block circuit diagram of FIG. 4.l Sim- TheY nariy, the wires 14s at the upper rea side of FIG. 6 are the recording traces indicated by the numbers 1 through Y and also to pairs 31 through'36. These 24 signals provide f or arrays in land geophysical work. Accordingly, from the foregoing remarks, it is to be understood that the present invention is to be limited only by the spirit and scope of the appended claims;

In'the claims: t,

l. A compound seismic survey cable apparatus comprising: Y

a terminal lcable section,

at leastone central cable section, means for connecting said central cable section to said terminal cable section;Y

a remote cable section, means for connecting said re mote cable section to one of said central cable sections;V Y l `irstfhydrophone groups connected to said centralcable section, said hydrophone groups being spaced apart The principles of the present in- Y s said first cable'section, and a second set of at least 12 hydrophone groups spaced apart at a distance approxirnately equal to 1/2 the spacingof saidi first set of six hydrophone groups, said second set being connected to 12 of Vsaid'30 circuits; and M* A a thirdcable section havingI at leastsix signal-'carrying tion to the free end of -said second cable section, and

`a third set of at least six hydrophone groups spaced a a apart by approximately the same distances as those in said first cable section, `Vand connected to said six circuits.

' 3. A compound oceanographic survey apparatus, comprising a central section of 24 hydrophone groups connected to a cable, six additional hydrophone groups connected to a remote section of the cable, an additional six hydrophone groups connected to a proximate section of thecable, thespacing of the additional hydrophone groups p being substantially greater than the spacing of the 24 hydrophone groups of said central section, and means for connecting all of the hydrophone groups to the proximate end of the cable.

4. A compound oceanographic survey cable apparatus l comprising:

a terminal cable section for connection to the survey vessel; v at least one central cable section having one end connected to said terminal cable section and a remote cable section connected to the other end of said central cable section;

. afirst spread of hydrophone groups connected to said central cable section, said hydrophone groups being spaced apart by a predetermined distance;

additional hydrophone groups connected to said terrninal and said remote cable sections, the spacing of said additional hydrophone groups being a multiple v of more than one of said predetermined distance; and

circuits, means for connecting the third cable secsaid terminal'and central. cable sections having more l leads than said remote section to carry seismic signals v from the distant hydrophone groups.

5. A compound oceanographic survey cable apparatus comprising:

a terminal cable section for connection to the survey vessel;

atleast one central cable section havingone end connected to said terminal cable section, and a remote cable section connected to the other end of said central cable section;

a first spread of hydrophone groups connected tosaid central cable section, said hydrophone groups being spaced apart by a predetermined distance; and s additional hydrophone groups connected to said terminal and said remote cable, the spacing of said additional hydrophone groups being significantly different l, l from said predetermined distance.

6. A compound oceanographic survey cable apparatus comprising:

a terminal cable section; at least one central cable section, means for connecting said central cable section to said terminal cable section; n Y a remote cable section, means for'connecting said remote cable section to one of said central cable sections; Y first hydrophone groups connected to said central cable section, said hydrophone groups being spaced apart by a predetermined distance; and a n additional hydrophone groups connected to said terminal and said remote cable sections, the spacing of said additional hydrophone groups being significantly different from said predetermined distance.

survey apparatus, comprising:

a plurality of hydrophones; v

means for supporting aser-iesr 'of said hydrophonesin a line in a body of Water, said series including a first spread of spaced hydrophone groups in one region alongv said line and additional more widely-spaced 7. A marine seismic hydrophone groups located in another ,region'along said line; l

means for recording a set of seismic signals from said first spread of hydrophone groups; and

additional means for simultaneously` recording'a set of seismic signals from'said more Widely-spaced hydrophone groups and selected hydrophone groups of said first spread.

` 8. A marine seismic survey cable apparatus comprising:

a terminal cable section for connection to a survey vessel;

at least one central cable section having one end connected to said terminal cable section and a remote cable section connected to the other end of said central cable section;

a first spread of hydrophone groups connected to said central cable section, said hydrophone groups being spaced Aapart by a predetermined distance; and

additional hydrophone groups connected to said terminal and said remote cable sections, the spacing of said additional hydrophone groups being a multiple of more than one of said predetermined distance.

` 9. A seismic survey apparatus comprising:

a plurality of detectors;

means for supporting a set of said detectors over a geographical area to be surveyed, said set including a first array of detector groups having a predetermined spacing in one region of said area, and additionaldetector groups spaced apart by distances which are greater than but commensurable with said predetermined spacing located in another region of s'aid area;

first recording means for recording a set of seismic signals from said first array of detector groups; and

additional recording means for recording a simultaneously a set of seismic signals from said additional detector groups and selected detector groups of said first spread array.

l0. A seismic survey apparatus comprising:

a plurality of detectors;

means for supporting a-set of said detectors over a geographical area to be surveyed, said set including a first array of spaced detector groups in one region of said area and additional more Widely-spaced detector groups located in another region of said area;

first recording means for recording a set of seismic signals from said first array of detectors; and

second recording means for simultaneously recording a set of seismic signals from said additional detector groups and selected detector groups of said first array.

1l. An oceanographic survey apparatus for producing electrical signals representing seismic data for recording comprising: a plurality of hydrophone groups and support means for supporting said hydrophone groups ina substantially linear array to survey a geographical area,

said support means additionally comprising means for i supporting a second spread of hydrophone groups in a second region of said linear array with the hydrophone groups of the second spread being spacedfarther apart from each other than. said predetermined distance,

and means associated With said support means for con- Y veying a first set ofelectrical signals from said first spread of hydrophone groups in said first region, and means associated with said support means for con- 1 1 veying a second set of electrical signals 'from hydrophones in said rst and second regions. v

l2. A seismic surveying apparatus comprising:

means for` supporting a rst spread of detector groups in a substantially linear array, the detector groups inV said array being spaced apart by a predetermined distance;

means for supporting additional detector groups substantially aligned With and adjoining said rst array and spaced apart by distances which are `greater than said predetermined distance; Y

means for producing a seismic shock near said arrays;

12 means for recording a set of seismic signals from said rst/ array and for recording an additional set of seismic signals from said additional'detector groups and from `selected detector groups in said rst array, 5` which form, with said additional detector groups an array more Widely spaced than said rst array.

References Cited in the le of this patent UNITED STATES PATENTS 10 `2,590,531 s f McLoad Mar. 25, 1952 

9. A SEISMIC SURVEY APPARATUS COMPRISING: A PLURALITY OF DETECTORS; MEANS FOR SUPPORTING A SET OF SAID DETECTORS OVER A GEOGRAPHICAL AREA TO BE SURVEYED, SAID SET INCLUDING A FIRST ARRAY OF DETECTOR GROUPS HAVING A PREDETERMINED SPACING IN ONE REGION OF SAID AREA, AND ADDITIONAL DETECTOR GROUPS SPACED APART BY DISTANCES WHICH ARE GREATER THAN BUT COMMENSURABLE WITH SAID PREDETERMINED SPACING LOCATED IN ANOTHER REGION OF SAID AREA; FIRST RECORDING MEANS FOR RECORDING A SET OF SEISMIC SIGNALS FROM SAID FIRST ARRAY OF DETECTOR GROUPS; AND ADDITIONAL RECORDING MEANS FOR RECORDING A SIMULTANEOUSLY A SET OF SEISMIC SIGNALS FROM SAID ADDITIONAL DETECTOR GROUPS AND SELECTED DETECTOR GROUPS OF SAID FIRST SPREAD ARRAY. 